An amateur looking at the harm principle

I'm aware of the gist of the harm principle, which is that people should be free to do whatever they want so long as it doesn't harm others. My question is... why? What is the problem that the harm principle is actually trying to solve? Is it just that this is the most sustainable moral code for a society to live by or is there a deeper train of thought that leads us to this being morally right?
I've got a few ideas about it, just wanting to run it by some more knowledgeable people.
First time on here- please try to not tear me apart for being an amateur too quickly.